Silylenediamido [(CH3)2Si(NTs)2(2-); Ts = p-CH3C6H4SO2] complexes of iridium: synthesis, structures and facile Si-N bond cleavage

The N,N'-bis(sulfonyl)diaminosilane TsdmsinH(2) (TsdmsinH(2) = (CH(3))(2)Si(NHTs)(2), Ts = p-CH(3)C(6)H(4)SO(2)) reacted with [Cp*IrCl(2)](2) (Cp* = eta(5)-C(5)(CH(3))(5)) in the presence of a base to give the coordinatively unsaturated (silylenediamido)iridium complex [Cp*Ir(Tsdmsin)] (2), which was further converted to the 18e adducts [Cp*Ir(Tsdmsin)L] (L = P(C(6)H(5))(3) (3a), P(OC(2)H(5))(3), CO); the reactions of 2 and 3a with water led to the formation of the imido-bridged dinuclear complex [Cp*Ir(micro(2)-NTs)(2)IrCp*] and the bis(amido) complex [Cp*Ir(NHTs)(2){P(C(6)H(5))(3)}], respectively.     

Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework

Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃]. ¹⁵N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms.     

Rhodium-catalyzed enantioselective [2+2+2] cycloaddition of diynes with unfunctionalized alkenes

A chiral rhodium complex catalyzed an enantioselective [2+2+2] cycloaddition of unsymmetrical diynes with norbornene, and tetracyclic products were obtained in good to excellent ee. The cycloaddition of a symmetrical diyne with styrene derivatives as coupling partners gave bicyclic products in good ee.     

Magnetic circular dichroism spectra of some 3-arylfervenulins and 3-aryltoxoflavins

The magnetic circular dichroism (MCU) spectra of some 3-arylfervenulins and 3-aryltoxo-flavins were measured. From the dependence of the Faraday B values of 3-arylfervenulins and 3-aryltoxoflavins on the Hammet σpara values, it is concluded that the substituent effects on the Faraday B values of 3-aryltoxoflavins are reverse to the substituent effects on the Faraday B values of 3-arylfervenulins and the reverse property of 3-aryltoxoflavins concerning the substituent effects rises from the contribution of the 1,5-dipolar structure of 3-aryltoxoflavins.     

SPIN-TRAPPING AND ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES AND POLYPEPTIDES IN AQUEOUS SOLUTIONS—I. PHENYLALANINE AND RELATED COMPOUNDS

Abstract— The direct UV photolysis of l -Phe and peptides containing l -Phe in aqueous solutions has been investigated at room temperature. The short-lived free radicals formed during photolysis were spin-trapped by t -nitrosobutane and identified by electron spin resonance. During the photolysis of l -Phe the decarboxylation and the deamination radicals were spin-trapped. For N-formyl and N-acetyl- l -Phe the decarboxylation radicals were observed. For dipeptides containing Phe the decarboxylation radicals were observed and in some cases the deamination radicals from the N-terminal residue were found. For the tripeptides Gly- l -Phe- l -Ala and Gly-Gly- l -Phe, the C-terminal decarboxylation radical was spin trapped; for l -Phe-Gly-Gly only the deamination radical of the N-terminal residue could be detected. However, for Gly- l -Phe-Gly, five different radicals were identified. The results of the spin-trapping experiments of the 260 nm photolysis of RNase-S-peptide, containing 20 amino acid residues, was interpreted in terms of a chain scission between the alpha carbon of the Phe residue and the adjacent carbonyl group.     

Intramolecular 1,3-dipolar cycloaddition of nitrile N-oxide accompanied by dearomatization

Intramolecular 1,3-dipolar cycloaddition of 2-phenoxybenzonitrile N-oxides to benzene rings, accompanied by dearomatization, formed the corresponding isoxazolines in high yields. The X-ray single-crystal structure analysis revealed that the reaction formed the cis-adduct as a single isomer. The substituents on the benzene rings markedly affected the reaction rate, yield, and structure of the final product.     

A Bacterial Chitinase Acts as Catalyst for Synthesis of the N‐Linked Oligosaccharide Core Trisaccharide by Employing a Sugar Oxazoline Substrate

A chitinolytic enzyme, chitinase A1 from Bacillus circulans WL‐12, was found to catalyze a glycosyl‐transferring reaction to form the N‐linked oligosaccharide core structure, Man(β1‐4)‐GlcNAc(β1‐4)‐GlcNAc, by employing Man(β1‐4)‐GlcNAc‐oxazoline as glycosyl donor. When the reaction was carried out in the presence of 20 v/v% acetone, the trisaccharide was obtained in 32% yield. It has been shown for the first time that a chitinase behaves like an endo‐β‐N‐acetylglucosaminidase in spite of low structural similarity between them.     

Syntheses of 3-acetoacetylaminobenzo[b]furan derivatives having cysteinyl leukotriene 2 receptor antagonistic activity

Novel 3-acetoacetylaminobenzo[b]furan derivatives having a modified triene system at the 3-position were synthesized starting with 3-aminobenzo[b]furans. The enol isomers, 3-[(3-hydroxybut-2-enonyl)amino]benzo[b]furans (), of the 3-acetoacetylaminobenzo[b]furans were obtained as stable isomers owing to formation of a hydrogen bonding between the enol hydroxyl group and the amidocarbonyl group. The planarity of the C-2 substituent through the C-3 side chain suggested the existence of a modified conjugational triene system in the enol compound. Cysteinyl leukotriene 1 and 2 receptor antagonistic activities for these compounds were evaluated. 2-(4-Cyanobenzoyl or ethoxycarbonyl)-3-[(2-cyano-3-hydroxybut-2-enonyl)amino]benzo[b]furans (, ) were moderately active.     

Mono(sulfido)-bridged mixed-valence nitrosyl complex: protonation and oxidative addition of iodine across the Ir(II)-Ir(0) bond

Treatment of [Cp*IrH(SH)(PMe3)] (Cp* = eta5-C5Me5) with [IrCl2(NO)(PPh3)2] in the presence of triethylamine yielded the sulfido-bridged Ir(II)Ir0 complex [Cp*Ir(PMe3)(mu-S)Ir(NO)(PPh3)], which further reacted with I2 and triflic acid to give the diiodo complex [Cp*Ir(PMe3)(mu-I)(mu-S)IrI(NO)(PPh3)] and the hydrido complex [Cp*Ir(PMe3)(mu-H)(mu-S)Ir(NO)(PPh3)][OSO2CF3], respectively.